The present invention relates generally to telecommunication systems, and relates more particularly to cable television systems and methods for isolating upstream ingress noise in a bi-directional cable system.
Cable communication systems typically comprise a head-end facility from which a plurality of main distribution lines emanate in a hub-and-spoke arrangement. Each main distribution line serves one or more local distribution networks each of which services a relatively small number (e.g., about 100 to 250) of individual subscribers. Each subscriber is connected with the main distribution line via a feeder line that taps into the main distribution line at a feeder tap location.
The coaxial cable portions of the cable system installed by the service provider generally have high-quality shielding to effectively prevent radio frequency noise from being picked up by the coaxial cable portions and transmitted through the system. However, at weak points in the network such as shield discontinuities, faulty connectors, and the settop terminals installed in the subscribers"" homes, radio frequency electromagnetic signals can leak into the cable. Ingress noise interferes with the desired signals transmitted over the network. There are various potential sources of ingress noise, including radio transmissions by two-way dispatch services, amateur radio transmissions, ignition noise from automobile engines, electric motors in household appliances, and the like. These emissions can be picked up by the coaxial cable via unterminated cable stubs in the subscriber""s home because the stubs tend to act as antennas.
In the early history of cable television, communications were strictly unidirectional, i.e., from the service provider to the subscribers. Accordingly, any ingress noise originating in a given subscriber""s premises could deleteriously affect that subscriber""s reception, but other subscribers were not affected by it. More recently, however, many service providers have begun to offer bidirectional services in which signals are also sent from the subscribers to the service provider. For example, cable television (CATV) service providers frequently offer impulse pay-per-view services where the subscriber may impulsively select an event for viewing and incur a charge. The pay-per-view service employs at least one data channel such as a telephone communication channel or an RF channel in an upstream (reverse) direction from the cable television subscriber to a cable television head-end for reporting service usage data. Other uses for a return path include internet access, active device status monitoring, power meter reading, alarm services, subscriber polling and voting, collecting subscriber viewing statistics, home shopping, and telephony.
Bi-directional cable communication systems usually utilize one frequency band for forward or downstream signals that are sent from the service provider to the subscribers, and a different frequency band for return or upstream signals sent from the subscribers to the service provider. These systems have typically used the higher of the frequency bands for forward transmission and the lower of the frequency bands for reverse transmission. Forward transmissions, for example, have been provided in the 54-550 MHz band. Reverse transmissions have generally been provided in the 5 to 40 MHz band.
Ingress noise can appear in either the upstream or downstream portion of the cable transmission frequency spectrum. Upstream ingress noise is particularly troublesome to cable service providers because the individual sources of ingress noise from various weak points in the system are added together and are amplified right along with the desired return signals being transmitted in the upstream direction.
Accordingly, the resulting ingress noise reaching the head-end can seriously interfere the reception of the desired return signals.
One method of reducing ingress noise from individual subscribers is to place a filter at the tap location of each subscriber that does not use upstream communication equipment. This limits the ingress noise sources to tap locations corresponding to homes that utilize the upstream communication path. However, since many of the services offered by CATV providers are interactive, a large proportion of the total homes served by the provider may utilize the upstream communication path. Therefore, this method of reducing ingress noise has limited benefits. Furthermore, where a large number of subscribers do not use the upstream communication path, the method requires a large number of filters, i.e., one for each such subscriber.
As an example of another technique that has been proposed for reducing ingress noise, U.S. Pat. No. 5,235,619 discloses a bi-directional CATV communication system which compensates for unwanted ingress noise by redundantly communicating each message from the settop terminals in the subscribers"" homes to the head-end over a plurality of frequencies and time slots. The head-end receives and compares the redundant transmissions to confirm the transmitted message. This system requires high-speed hardware to be located both at the head-end and at each settop terminal, resulting in a system that is inefficient and expensive.
The above needs are met and other advantages are achieved by the present invention, which provides a system and method for providing bi-directional communication between a service provider and a plurality of subscribers, in which a single return-path filter is employed for reducing ingress noise originating from a plurality of subscribers. More particularly, the invention provides a telecommunications system for bi-directional communication services between a service provider and a plurality of subscribers wherein return signals associated with the bi-directional communication services utilize a first frequency band. The system includes a head-end facility for sending forward signals from the service provider and for receiving return signals from one or more of the subscribers; at least one main distribution line extending from the head-end facility; a plurality of feeder lines for serving the plurality of subscribers, each feeder line being connected to the main distribution line at a feeder tap location; a local distribution amplifier unit disposed in the main distribution line between the head-end facility and the feeder tap locations, the amplifier unit including forward-path amplifier circuitry for amplifying forward signals traveling from the service provider to the subscribers, the amplifier unit further including return-path amplifier circuitry for amplifying return signals traveling from one or more of the subscribers to the service provider; and a return-path filter disposed between the head-end facility and the feeder tap locations, the return-path filter being operable to attenuate signals outside the first frequency band while allowing signals within the first frequency band to pass substantially unattenuated, whereby the return-path filter reduces transmission of ingress noise from each of the plurality of subscribers.
In a preferred embodiment of the invention, the amplifier unit includes a receptacle for receiving an optional plug-in electrical device, and the return-path filter is installed in the receptacle. For example, CATV systems frequently include a distribution amplifier for a relatively small number (e.g., about 100 to 250) subscribers, which may constitute a neighborhood or the like. The distribution amplifier often includes ports or slots into which optional electrical devices such as simple flat-loss attenuator pads can be inserted into the return path of the amplifier if desired. In other cases, the ports or slots are used for adding thermal compensation units to the amplifier to stabilize the gain of the return-path amplifier over a range of temperatures. In accordance with the present invention, the return-path filter is configured to be received in such a port or slot of the distribution amplifier. Therefore, rather than having to install multiple return-path filters at each subscriber location and incurring the associated installation costs, one filter can be used at the amplifier to achieve the same results.